Field of the Invention
The present invention relates to a receiver in a digital radio communication system, and more specifically to a single conversion receiver and a communication system.
Background Art
FIG. 3 is a block diagram illustrating the structure of a conventional single conversion receiver.
The conventional single conversion receiver includes an input terminal 31 to which a received RF signal is input, a bandpass filter 32, an RF amplifier 33, a mixer 34, an IF amplifier 35, mixers 36 and 37, a channel selection control circuit 38, an oscillator 39, a 1/N frequency dividing circuit 40, a ¼ frequency dividing circuit 41, and lowpass filters 42 and 43.
The received RF signal is input to the RF amplifier 33 through the bandpass filter 32, where only a desired band of RF signal is amplified. The center frequency of the bandpass filter 32 is controlled by the channel selection control circuit 38. The oscillator 39 outputs a local signal. The frequency of the local signal is controlled by the channel selection control circuit 38 according to the frequency of the RF signal. The mixer 34 multiplies the RF signal output from the RF amplifier 33 and the local signal output from the oscillator 39 together. The output of the mixer 34 is amplified by the IF amplifier 35, and input to the mixers 36 and 37.
The local signal output from the oscillator 39 is set to be 4N/(4N±1) times the frequency of the RF signal (where N is an integer). The frequency of the output signal from the mixer 34 is 1/(4N±1) times the frequency of the RF signal. The local signal output from the oscillator 39 is input to the 1/N frequency dividing circuit 40 and the ¼ frequency dividing circuit 41. The frequency of the output signal becomes 1/(4N±1) times the frequency of the RF signal, and is input to the mixers 36 and 37. Thus, quadrature detection is performed by the mixers 36 and 37, and respective output signals are output to the lowpass filters 42 and 43. Then, the lowpass filters 42 and 43 remove unwanted harmonic components generated by the multiplication and noise components outside of the receiving band to output baseband signals from respective output terminals (for example, see Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-168934